The invention is particularly concerned with display apparatus for use in head-up display systems, that is to say, systems of the kind in which a display is generated for projection onto a partially-transparent optical element located in a line-of-sight from an observer such as to provide an image of the display against the background of the external scene through the element. Such systems are used in, for example, aircraft, the display being provided by a cathode-ray tube and involving symbols that in the image seen in the partially-transparent element, are positioned against the background of the external scene through the aircraft windscreen to give indication of such factors as aircraft attitude and flight-path. The optical element used in such systems is generally in the form of a reflector provided by a transparent glass plate coated with a semi-transparent metallic layer.
Because of the limited space available within an aircraft cockpit, and hence the restrictions imposed on the size of apparatus used in the cockpit, it has been difficult in the past to provide head-up display systems with large exit pupils and large fields-of-view. The exit pupil and field-of-view of such systems are generally dictated by the size of the display screen and the optical system used to project the image of the screen into the observer's line-of-sight. A large exit pupil is desirable since it enables the observer to see the display over a greater range of head positions and hence gives him greater freedom of movement.